Centrifugal air classifiers



March 5, 1968 Filed Dec. 8, 1965 P. MEYER ETAL CENTRIFUGALAIRCLASSIFIERS 4 Sheets-Sheet 1 V 17 3 I 8 l 2 I? 37 I I. 7, I u

March 5, 1968 MEYER ETAL 3,371,783

CENTRIFUGAL AIR CLASSIFIERS Filed Dec. 8, 1965 4 Sheets-Sheet 2 i mwlgob2 .ar mama/7M March 5, 1968 P. MEYER ETAL 3,371,783

CENTRIFUGAL AIR CLASSIFIERS 4 Sheets-Sheet 5 Filed Dec. 8, 1965 4Sheets-Sheet 4 Filed Dec. 8, 1965 United States Patent 3,371,783CENTRIFUGAL AFR CLASSIFIERS Paul Meyer, 19 Paget Road, London, N. 16,England, and Geoffrey Daniel Humphreys, Woodbridge, Ash Road, Hartley,near Dartford, Kent, England Filed Dec. 8, 1965, Ser. No. 512,309 Claimspriority, application Great Britain, Dec. 11, 1964, 56,647/64 18 Claims.(Cl. 209-144) This invention relates to centrifugal air classifiers forground or other solid particles of varying size and/or density.

Rotary air classifiers are known in which a stream of air or othercarrier fluid is induced to flow from the periphery of a rotor spirallyinwards to emerge at the eye thereof. Material to be classified iseither introduced into the rotor at some position between the peripheryand the eye or enters the rotor at its periphery together with thecarrienfluid. In the rotor, the suspension is subjected to an interplayof centrifugal and drag-forces such that, as many as pOsSible of theparticles below a required cut size, the fines, emerge from the eye ofthe rotor while as many as possible of the oversize or rejects areejected at the periphery of the rotor.

To further this action, rotors are fashioned with shrouds such that thedepth of the path of the air increases towards the centre, i.e., withdecreasing radius, centrifugal force decreases to such a degree that,for particles of a given size, the radial drag-force component and thecentrifugal force are in the same ratio over the whole flowpath betweenperiphery and eye of the rotor. While for the fines, this ratio exceeds1, and for the rejects it is less than one, the ratio is approximatelyunity for the cut size. For this reason there is a tendency for out sizeparticles to collect in the rotor, particularly if the rotor is mountedon a substantially vertical axis and is given a flat or inward anddownward sloping bottom shroud.

Furthermore, while the above constant ratio type of rotor makes a highdegree of separation possible in principle, this in practice is limitedby the means applied to impose the rotation on the air-stream with itsdust entrained to a greater or smaller degree, as well as by meansemployed particularly in rinsing the rejects, to eliminate agglomeratedfines therefrom.

Where the particles are introduced as a powder-flow into a constantratio type rotor at some position between the periphery and the eye ofthe rotor (powder fed type), the minimum air quantity, to make efilcientseparation possible will be pumped through the rotor to keep down thepower input, both on pumping and on turning the rotor. With the constantratio type rotor, which makes efiective separation possible at high dustburdens, this leads to rotors which are of low depth at their periphery,and hence are compact and easy to manufacture.

Where the particles enter the rotor periphery together with the air(air-swept type), usually coming directly from some grinding machine,the dust burden in the air is usually much less than a constant ratiorotor could handle. In order not to impose too high a pressure loss onthe air-stream, the radial inward speed of the air must be kept withinlimits so that rotors of very considerable depth at the eye result,which are somewhat clumsy and expensive. At best, a compromise betweensharpness of cut, pressure-loss of the air, and power input into therotor, at some values less than optimum is called for.

It is the object of this invention to reduce or to eliminate the abovedrawbacks for constant ratio type air clas- 3,371,783 Patented Mar. 5,1968 sifiers, respectively of the powder fed and of the airswept types.

It is a further object of this invention to provide a classifier adaptedto be fed directly from air transport systems with very high powder toair ratios, such as air-slide and similar systems.

According to the present invention in a rotary air classifier having arotor counted to revolve about a vertical axis within the casing, therotor comprising a top plate and a bottom shroud plate and theclassifying zone defined between said plates being subdivided by radialblades, the shroud plate is sloped downwardly towards the centre topermit gravity to assist in rejection of particles near cut size and notrejected by centrifugal force exceeding the drag exerted by the carrierfluid, and an opening or openings is provided at the bottom of theshroud plate for discharge of the cut-size particles.

In carrying out the invention as applied to a rotary air classifier,which is swept by the carrier fluid, the classifier comprises acylindrical casing including an upper portion and a lower portion; arotor mounted to revolve within said lower portion; an inner shellwithin the casing and having a tangentially disposed inlet fordust-laden-fluid in order to set up a cyclone efiect within the shell,means connecting said shell and rotor by which material is introduced tothe rotor as it revolves; the upper portion of the casing having acentral duct for discharge of carrier fluid and fines from the rotor andthe lower portion having a bottom opening for discharge of classifiedparticles; and said rotor comprising a top plate and a bottom shroudplate, the classifying zone defined between said plates being subdividedby radial blades, said zone being open at its perimeter to the lower endof the admission passage for the dust-ladernair; the shroud plate beingsloped downwardly towards the centre to permit gravity to assist inrejection of particles near cut size and not rejected by centrifugalforce exceeding the drag exerted by the carrier fluid and means forimparting rotation to the rotor.

This invention will now be described with reference tot he accompanyingdrawing in which:

FIGURE 1 is a sectional elevation of a constant ratio type powder-fedclassifier according to this invention, and

FIGURE 2 is a plan view of its rotor.

FIGURE 3 is a sectional elevation of a second em bodiment of powder-fedclassifier.

FIGURES 4 and 5 are a sectional elevation and a plan of air-sweptclassifier.

FIGURES 6 and 7 are a sectional elevation and a plan of a furtherembodiment of air-swept classifier.

FIGURES 8 and 9 are a sectional elevation and plan of a furtherembodiment of an air-swept classifier.

Referring first to FIGURES 1 and 2, the classifier comprises a rotor 2fast on a central shaft 3, mounted to revolve inside a stationarycasing.

The rotor consists of a bottom plate or table having radial blades 5,surmounted by a top plate or cowl so shaped that the cross section ofthe rotor chamber increases in depth from the perimeter to the centre.

Particles of material to be classified, when they have entered therotor, are subjected to the action of centrifugal force, the magnitudeof which force is directly proportional to the distance of the particlesfrom the centre of the rotor. In existing classifiers, it has been thepractice to build the rotor to Careys design (see British Patent No.490,393) to ensure that the inward radial velocity of the fluidprogressively diminishes towards the centre of the rotor table in such amanner that at all points it just balances the centrifugal force actingupon a particle of the size at which a cut is desired. In consequencethe particles of the size at which a separation is required, describecircles and move neither outwardly nor inwardly. Finer particles moveinwardly in spirals and are drawn away with the fluid to dischargethrough an orifice at the top of the casing. Coarser particles moveoutwardly in spirals and fall into the bottom of the casing and therewithdrawn.

Slits 14 in the blades enable the powder to pass from a compartmentbetween one pair of blades to the next where it is against subjected tothe action of the air stream in the adjacent compartment.

We have now found that by providing an inwardly and downwardly directedslope in the bottom plate of the rotor that it enables the cut sizeparticles to be separately extracted and furthermore that the classifiercan be operated more satisfactorily at lower rotor speeds.

In the embodiment of FIGURES 1 and 2, the classifier comprises anannular casing having a conical lower section 22, terminating at itslower end in a discharge orifice 15. Extending into the lower section22, is a cylindrical housing 23, within which is rotatably mounted theshaft 3 which carries the rotor.

The rotor, according to this invention, comprises a flat top plate orcowl 12 and a bottom shroud 24, which is curved downwardly towards thecentral axis so as to provide a central bottom opening, which is partlyclosed by a circular plate 26, plate 26 being spaced apart from thebottom opening so as to provide an annular discharge slot 27 forextraction of the cut size particles as hereinafter described. Insteadof the slot 27, the plate 26 may be joined to the bottom of the shroudand a series of holes provided for the same purpose.

Connecting the top plate 12 to the shroud 24 are the series of radialblades 5, these blades, as in previous constructions, being formed withvertical slits so as to permit the material to be classified, to movefrom the compartment on one side of the blade to that on the other.Immediately above the top plate 12 of the rotor is an annular plate 21which at its inner end connects with a duct 19, thereby enclosing thelower section 22 of the casing within which the rotor operates andseparating it from the upper section 20. The duct 19 is open at itslower end to the eye of the rotor and provides a discharge for thecarrier fluid and fines, its upper end opening into a volute chamber 35,from which leads an outlet pipe 39 for the air and fines that have beenseparated.

Extending downwardly through the chamber and duct 19 is a tube 32 whichat its upper end connects with a hopper 33 for introduction of materialto be classified, the lower end of the tube opening into the centrespace of the rotor. Certain of the blades 5 are cut away so as toprovide radially projecting entry passages 36 for feeding the powderedmaterial into the classifying zone.

The tube 32 rotates with the rotor and a bearing 17 together with a seal37 is provided at the top part of the casing within which the tuberevolves, the hopper 33 being attached to the bearing housing.

Carrier fluid, i.e., air is admitted through a pipe 31 which opens intoan annular passage 34 in the upper section of the casing above the coverplate 21. The inner wall 37 of passage 34 is constructed in the form ofa volute, the in-coming air passing downwardly through a bottom opening38 so that the air enters the rotor at its perimeter where the distanceseparating the top plate 12 and shroud 24 is a minimum.

By constructing the chamber wall 37 as a volute with the point of entryat the beginning of the spiral, it has the advantage that the airpressure within chamber 34 can be maintained at an even balance over theentire perimeter. Likewise the chamber 35 communicating with thedischarge pipe 39 is in the form of a volute.

As can be seen from FIGURE 2, the passages 36 for entry of powder leaveopen a sufficient area between each passage so that air carrying thefines, which passes 4 into the eye 28 of the rotor, has an unhinderedegress to the chamber 35.

To prevent any direct passage of air by-passing the outside edge of therotor, a skirt device in the form of upper and lower, i.e., axiallyspaced skirt elements are provided on the shroud plate 24 of the rotor.One skirt 40 is fixed to the outer perimeter of the plate 24 and theother skirt 40' is fixed to its lower end. By arranging for theclearance between each skirt element and the casing 22 to be a minimumair-leakage past the rotor is, in practice, virtually eliminated.

In the construction according to this invention, those rejects at ornear cut size which are thrown against the lower shroud and not ejectedunder an excess centrifugal force, move down the shroud under gravityand are ejected through the lower slot or opening 27 by centrifugalforce. This has the advantage that classification can take place atlower centrifugal fields and correspondingly lower radially inwardair-speeds. This permits savings in classifier drive power and onair-power which are substantial on big units with large throughputs.

Referring now to FIGURE 3, there is shown a further embodiment whereinthe rotor is suspended from a hollow shaft 43 carried in bearings in theupper section of the casing, the shaft 43 supporting the feed hopper 33.The rotor, as in the FIGURE 1 arrangement, comprises a top plate 12 andan inwardly curved shroud 24, extending between which are blades 5, theeye of the rotor in this case, facing downwardly and dischargingdirectly into a pipe 41 which curves sidewardly out of the rejects cone22 or lower section of the casing. It will be seen that a gap isprovided between the bottom opening of the rotor and the outlet pipe 41.This gap, which is similar to the discharge opening 2'7, forms asecondary outlet for particles which have been pushed against the shroud24 by centrifugal force and/ or by gravity and are running down it dueto gravity.

The provision of the skirts 40 and 40' together with the vanes 29,limits the volume of air which can bypass the rotor: Furthermore, theprovision of extension blades 42, on the underside of the skirt plate40, which serve to maintain the rotation of any such air look at rotorspeed, together with the existence of the reject slot 27' makes any kindof seal unnecessary at this position.

Material to be classified enters the hollow shaft 43 through the hopper33 and passes directly into a conical space 48' provided between acoverplate 48 and the top plate 12 and then is thrown outwardly when itfalls through openings 47 in the top plate to enter the rotor, theseopenings being in a position about half way along the classifying zone.

The hollow shaft 43 is fixed into a conical section 48 of the coverplate48 which, in its turn, is fixed to the top plate 12 of the rotor justoutside the feed slots 47.

Vanes 49 are fixed to the top plate of the rotor beyond the perimeter ofthe cover plate 48, and the main casing 22 has a cylindrical rim 50which overlies the outside edge of the vanes 49. With the casing 22having a fairly small clearance to the cover plate 48 and the vanes 49sweeping the space between the two restrictions, an effective seal isformed keeping down air-leakages over the top of the rotor, withoutinvolving really tight, and hence expensive, clearances anywhere.

In operation, powder to be classified is introduced by means of thehopper 33 and feed shaft 43 and thence distributed on the rotating plate12, passing into the rotor through slots 47. With air under pressure orsuction being distributed through volute 34, classification takes placein the rotor as described above, with most rejects emerging from theperiphery of the rotor and falling down the rejects cone 22, through theclearances between shirt 40 and the plate 40' and the cone.

Air and fines emerge through pipe 41 while what rejects that pass downthe inclined shroud will be ejected through a secondary rejects exitslot 27. No fines will find their way out through the slot because of aleakage flow of clean air past the underside of the rotor, entering exitpipe 41 through slot 27'. This leakage flow, conversely, will haveinsufficient speed to carry any rejects with it into pipe 41, therejects moving outwards under centrifugal force.

To avoid air leakage between the feed hopper and the classifying zone ofthe rotor which, if excessive, would upset classification, the powderfeed may be introduced through a rotary valve or equivalent device-notshown on the drawing.

Referring now to FIGURES 4-9, there is shown the application of thisinvention to an air-swept classifier. In the arrangement shown inFIGURES 4 and 5, the outer casing consists of an upper cylindricalsection 75 surmounting a lower conical section 76 which is closed at itslower end by a base plate 77. The rotor comprises a flat top plate 62,radial blades 63, a curved bottom plate or shroud 67 and a bottom plate65, the whole being mounted for rotation on a shaft 3 similar to theembodiment of the invention illustrated in FIGURES 1 and 2.

Material to be classified enters the rotor at its perimeter and therejects pass out through the perimeter opening and through a gap 69between the plate 65 and the shroud 67 whence they are dischargedthrough a rejects outlet 78. Removal of air and fines is upwardlythrough a pipe 71 and into a volute 73.

Dust-laden air enters through an inlet pipe 79 (FIG- URE 5) which opensinto a cylindrical shell 80 which operates as a cyclone and is open atthe top to allow air from the centre to pass first upwardly and thendownwardly through an annular passage 80" between shell 80 and thecasing 75 when it enters the perimeter of the rotor as denoted by thearrows. The bottom of the shell Sil is bounded by an annular plate 68,which terminates short of the inner surface of the shell so as toprovide a peripheral slot 86 for dust falling down the iner wall surfaceof the shell into the preclassifying zone, some air being introducedwith it. It should be noted that the lower end of the shell 84 extendsbeneath plate 68 so as to direct the down fiow of air through slot 36across the rotor where it encounters the main stream that descendsthrough passage 80: this air stream causes the dust particles toaccelerate radially inwards of the rotor, the dust particles, like theair already having a substantial circumferential velocity.

Sealing skirts 82 and 83 and a number of vanes 67 are provided as in theFIGURE 3 embodiment.

In operation the majority of particles are swept into the centre of therotor although some of the heavier ones will fall down and be directedby the skirt 82 directly into the reject cone 76 and pass into theoutlet 78.

In practice, it has been found that a deep rotor with a shortclassifying section gives the right order of airspeed to keep pressurelosses low. The rejects are rinsed by the action of the air-stream onthe curtain of material in the zone outside the outer circumference ofthe constant ratio rotor.

FIGURES 6 and 7 show a modification of the airswept classifier ofFIGURES 4 and 5 in which the shell 80, which functions as a cyclone,incorporates a lower conical section 87 so as to direct the dustparticles through an opening 86 and thence directly into theclassification zone of the rotor through an annular opening 86 in thetop plate 63 of the rotor. Drive to the rotor shaft may be byconventional means. In the embodiment shown in FIGURE 6 drive isdirectly from a motor M through a bevel pinion train of gears B.

It will be seen that the plate 63' of the rotor, as a result, hasslightly greater radial depth and consequently the heavier rejects,together with all the dust which passes directly via 86-86 into therotor, are rinsed in the rotor itself. It also follows that the casingas well as the inner shell can be of reduced diameter.

The air classifier shown in FIGURES 8 and 9 operates in a manner similarto that of FIGURES 6 and 7 but with the difference that the dischargepipe 71 for the air and fines is provided with valve plates or similarmeans in its wall providing a by-pass from the entry of the cyclonechamber 80 to the discharge pipe. In the embodiment illustrated, twoValve plates 9b are fitted to the outer perimeter of the pipe so as topermit of hinging movements about axes extending parallel to the axisand the tube so that by varying the opening of the valve plates, agreater or smaller amount of air can be bypassed directly into thedischarge pipe 71. It will be noted that position of the by-pass valvesW is in the upper portion of the chamber so that by-pass air will onlycontain superfines, the passage of which through the constant ratiorotor would only involve waste of power both on the air and on the rotordrive.

This enables the design of the constant ratio rotor itself toapproximate much more to the .flat shape of the powder-fed designs. Thisamounts to a considerable manufacturing simplification and saving ofspace compared to the deep rotors of air-swept designs feeding all theair through their rotors.

If desired the valve means 90 is regulated automatically through someregulating device indicated diagrammatically at 91, for instance, so asto maintain a constant air flow through the rotor with varying pressuredrops across the rotor.

What is claimed is:

1. A rotary air classifier including a casing provided with an inlet formaterial to be classified and a discharge outlet for classifiedparticles, and a rotor mounted to revolve about a vertival axis withinthe casing, said rotor comprising a top plate and a bottom shroud plate,the classifying zone defined between said plates being subdivided byradial blad-es, said rotor having a central orifice for discharge offluid and fines, said zone being open at its perimeter for admission ofcarrier fluid from the casing, said shroud plate being sloped downwardlyand towards the centre to permit gravity to assist in rejection ofparticles of approximately cut size and not ejected by centrifugal forceexceeding the drag exerted by the carrier fluid, said shroud platehaving at least one central opening for ejection of the rejectedparticles, means for supplying carrier fluid to the casing and means forimparting rotation to the rotor.

2. A rotary air classifier including a casing provided with an inlet formaterial to be classified, a discharge outlet at the bottom forclassified particles and a second discharge outlet for the carrier fluidand fines and a rotor mounted to revolve about a vertical axis withinthe casing, said rotor comprising a top plate and a bottom shroud plate,the classifying zone defined between said plates being subdivided byradial blades, said zone being open at its perimeter to admission ofcarrier fluid from the casing, means connecting said inlet to the rotorfor feeding raw material into the classifying zone, said shroud platebeing curved inwardly and downwardly towards the centre to permitgravity to assist in the ejection of particles of approximately cut sizeand not rejected by centrifugal force exceeding the drag exerted by thecarrier fluid, said shroud plate having at least one central opening forejection of the rejected particles, means for supplying carrier fluid tothe casing and means for imparting rotation to the rotor.

3. A rotary air classifier as claimed in claim 2 comprising a rotorhaving a central opening, said opening being closed at its lower end bya bottom plate spaced from the bottom shroud plate so as to provide anannular slit or slits and means including a central duct open at one endto the second discharge outlet and connecting at its other end with anopening in the top plate of the rotor, through which the carrier fluidand fines are discharged and tube means extending through said duct forconveying material to be classified from the inlet in the casing to therotor.

4 A rotary air classifier as claimed in claim 3 in which the tube meansis provided at its lower end with branch passages opening in to theclassifying zone.

5. A rotary air classifier as claimed in claims 2 wherein the rotor isprovided with a circumferentially extending skirt device, said devicehaving minimum clearance with the wall of the casing.

6. A rotary air classifier as claimed in claim 5 in which the skirtdevice comprises axially spaced skirt elements carried by the shroudplate.

7. A rotary air classifier as claimed in claim 6 in which radial vanesare provided on the outside of the shroud plate.

8. A rotary air classifier as claimed in claim 6 in which the top plateof the rotor is provided with radial blades to minimize air leakage.

9. A rotary air classifier including a cylindrical casing having aconical rejects section in which is a discharge outlet for classifiedparticles, said casing being provided with an inlet for material to beclassified and an inlet for carrier fluid, said classifier comprising adrive shaft mounted to revolve about a vertical axis concentric withthat of the casing, a rotor fast on said shaft and including a top plateand a bottom shroud plate, the classifying zone defined between saidplates being subdivided by radial blades, said classifying zone beingopen at its perimeter for inflow of carrier fluid from the casing, saidbottom shroud plate sloping downwardly to the centre to permit gravityto assist in the ejection of particles of approximately cut size and notrejected by centrifugal force exceeding the drag exerted by the carrierfiuid, said shroud plate having at least one central opening forejection of the rejected particles into said rejects section, meansconnecting said inlet to a source of carrier fluid and means forimparting drive to said shaft.

it). A rotary air classifier as claimed in claim 9 in which the rotorcomprises a bottom shroud plate having a central opening and the casingincludes an outlet pipe for fluid and fines, said pipe extending throughthe rejects section and registering at its inner end with the opening inthe shroud plate, said end being spaced from said opening to provide anannular slot for discharge of rejects.

11. A rotary air classifier, which is swept by the car rier fluid, saidclassifier comprising a cylindrical casing including an upper portionand a lower portion; a rotor mounted to revolve within said lowerportion; an inner shell within the casing and having a tangentiallydisposed inlet for dust-laden-fiuid in order to set up a cyclone effectwithin the shell, passage means connecting said shell and rotor by whichmaterial is introduced to the rotor as it revolves, the upper portion ofthe casing having a central duct for discharge of carrier fluid andfines from the rotor and the lower portion having a bottom opening fordischarge of classified particles; said rotor comprising a top plate anda bottom shroud plate, the classifying zone defined between said platesbeing subdivided by radial blades, said zone being open at its perimeterto the lower end of said passage means for the dust-laden-fiuid; saidshroud plate being sloped downwardly towards the centre to permitgravity to assist in the ejection of particles of approximately cut sizeand not rejected by centrifugal force exceeding the drag exerted by thecarrier fluid, said shoud plate having at least one central opening forejection of the rejected particles, and means for imparting r0- tationto the rotor.

12. A rotary air classifier as claimed in claim 11 Wherein the rotor isprovided with a circumferentially extending skirt device, said devicehaving minimum clearance with the wall of the casing.

13. A rotary air classifier as claimed in claim 11 in which the shell isspaced from the casing to provice an annular flow passage whose lowerend is situated opposite to the preclassification zone of the rotor.

14. A rotary air classifier as claimed in claim 13 in which said innershell has a bottom plate in which are openings for ejection of therejected particles providing direct access to the preclassification zoneof the rotor.

15. A rotary air classifier as claimed in claim 11 in which the innershell has a bottom plate in which is an annular opening registering withan annular opening in the top plate of the rotor, by whichdust-laden-air is introduced to the classification zone of the rotor.

16. A rotary air classifier as claimed in claim 15 in which the bottomplate includes a conical portion to direct dust particles through theopening giving access to the rotor casing.

17. A rotary air classifier as claimed in claim 11 comprising adischarge duct having valve means providing a by-pass between thecyclone chamber within said shell and the discharge duct.

18. A rotary air classifier as claimed in claim 17 in which the valvemeans consists of an opening in the wall of the duct, a valve plate bywhich the opening normally remains closed, and means for adjusting theposition of said valve plate to vary the size of the opening.

References (Iited UNITED STATES PATENTS 2,199,015 4/1940 To-ensteldt209144 X 2,276,761 3/1942 Carey 209-l45 X 2,338,779 1/1944 Mutch 2091443,989,595 4/1963 Kaiser 2G9--144 FOREIGN PATENTS 222,989 8/1962 Austria.

FRANK W. LUTTER, Primary Examiner.

TIM R. MILES, Examiner.

1. A ROTARY AIR CLASSIFIER INCLUDING A CASING PROVIDED WITH AN INLET FORMATERIAL TO BE CLASSIFIED AND A DISCHARGE OUTLET FOR CLASSIFIEDPARTICLES, AND A ROTOR MOUNTED TO REVOLVE ABOUT A VERTIVAL AXIS WITHINTHE CASING, SAID ROTOR COMPRISING A TOP PLATE AND A BOTTOM SHROUD PLATE,THE CLASSIFYING ZONE DEFINED BETWEEN SAID PLATES BEING SUBDIVIDED BYRADIAL BLADES, SAID ROTOR HAVING A CENTRAL ORIFICE FOR DISCHARGE OFFLUID AND FINES, SAID ZONE BEING OPEN AT ITS PERIMETER FOR ADMISSION OFCARRIER FLUID FROM THE